I have the RT-615s. They are amazing so far. I’ve put 2000 miles on em. 225/45/17 on my Mazda 6. Corners I couldn’t take at 45 with my stock tires which made me slide to the side, now hug the road and feel very firm and strong at 58. Onramps are fun too, even though I’m probably pushing it… but, they dont screech, they dont slide, they just hold the road.
I’m happy with my purchase.
Those were my two choices as well, but I decided to have a winter and summer set of tires, so… the Proxes 4 weren’t needed anymore 
Edgeracing.com for both. Great prices, unless you can find cheaper somewhere else. I couldn’t.